A steam valves is designed to keep the boiler or engine properly operating. It is important to know what these valves look like and how they work.

Types of steam valves

Each engine or boiler uses a different type of steam valve. The type of steam valve used depends on the type of vessel or engine used.
The simplest type of valve is the block valve. This type has two equal-sized flaps on each side, one to open and one to close.
These are used in simple engines. They’re not suitable for continuous operation, but they can be removed to allow the vessel to be brought up to operating temperature quickly.
Blocks are also used to make fire pumps and steam locomotives. The block valve is designed to work with a coil spring to restrict the flow of steam when the valve is closed, so that the pressure does not build up.
A conventional steam locomotive’s boiler will have a button valve. The “button” on this valve operates both the closed and open parts of the valve, and can be opened by two separate levers. The “button valve” system is normally found on ships, where it is necessary to increase steam pressure quickly.

Steam locomotives

The main valve is called a side-valve. The steam must pass through a lubricating tube called a sleeve. This tube makes the steam feel cool to touch. The valve can be opened and closed by the handle, but it cannot be opened more than three-quarters of the way.
Older steam engines can have a steam blast and steam tremble valve. This was an early form of steam valve, but has been replaced by the button valve. The steam blast provides pressure to open and close the valve. The steam tremble valve provides a ‘fog’ or mist effect. It reduces the amount of steam produced by the steam blast.

Choke valves and rain valves

Each type of steam valve has its own method of operation.
The choke valve is sometimes referred to as the ‘push and tug valve’. It has a spring for when the valve is closed, and a bulb that operates it when the valve is open. The only thing to remember with this type of valve is that steam must be allowed to escape in both directions at the same time. If it is shut completely, then air will be forced into the boiler or engine. This will quickly build up a pressure that will crack the boiler or engine, and even could be fatal to the person operating it.

A steam blast has been given off

To open the choke valve, the steam is passed between the two sides of the valve, and a pressure is applied to the steam. This closes the valve. This can be opened by turning the handles, or by the hammering of a bolt on the side of the valve. The choke valve is sometimes referred to as the ‘push and tug valve’.
Choke valves are typically used on stationary engines and boilers. A steam locomotive will usually have a straight choke valve.
The steam in the boiler must be able to travel through the valve either into the cylinder, or through the cylinder to the burner, and back to the cylinder and then the boiler. A steam locomotive uses a piston to connect to both the cylinder and the burner. The piston connects to the choke valve and forces the steam through the cylinder or burner, leaving enough for it to re-enter the cylinder or burner before it leaves through the choke. A steam locomotive must allow the steam to travel through the valve a full turn before closing it again, otherwise there will not be enough steam available for the piston to start the engine.

A steam locomotive steam blast

A steam locomotive will not have a steam blast or steam tremble valve. The steam only gets into the boiler if the piston is in the cylinder, or if the piston and valve are connected to a steam blast, steam tremble or pressure relief valve, called a valve of some type.
A pressure relief valve is a steam blast valve, and the steam escapes in the same direction as the steam blast. Steam is released at the same pressure as the steam blast, but it is ejected back through the same valve.
Steam engines, boilers and boilers in ships are all connected to one or more steam pressure relief valves. These are basically the same as the steam pressure relief valves that an engine might use, but some ships may use different types to meet the requirements. A ship will have a variety of valve types. Each type has its own method of operation. These valve types are most often the most difficult to operate. In the case of a pressure relief valve, they must be kept open at all times.
Some steam locomotives use a mixture of steam pressure relief valves. Some steam locomotives use steam pressure relief valves that have an integral shut off valve. This type of valve is often used in stationary engines, but the shut off valve is also commonly used in a boiler or a steam engine or steam engine firebox. When the steam pressure is too low, the engine could blow up or damage the firebox. This type of valve has an integral steam blast shut off valve. The steam blast will cease automatically when the shut off valve opens.
Many steam locomotives and boilers use steam pressure relief valves without an integral shut off valve. These valves usually require a worker to manually shut off the steam pressure before the steam pressure reaches a dangerous level.
Steam locomotives, boilers and boilers in ships are all connected to one or more steam pressure relief valves.
After leaving the cylinders of a steam locomotive, steam is usually redirected to the chokes.
An enormous steam blast or steam tremble valve. This is a very large steam blast valve used in some large steam powered locomotives. The steam pressure may exceed 500 pounds per square inch.
Note that in some engines, the side of the choke valve may be operated manually by a worker, for steam pressure relief.
Steam powered trains usually have a steam pressure relief valve on the choke. This valve will not operate when the steam pressure is below a safe level. If the steam pressure rises above this level, then the valve will open, and the steam blast will let loose and blow the throttle open. The choke valve must be well placed to direct the steam blast at the choke. The steam blast will drive the valve back and forth, until the valve opens.
Below is a picture of the valve as it is used to start up a steam locomotive.
In this image, the steam pressure is just above the limit. The blue trace is the pipe that carries the steam to the engine. The black line is the steam valve, and the red line is the choke valve.
Here is the steam valve or steam trap opening. The steam blast, which is shown running to the right, will drive the valve. It will increase the pressure in the pipe. As steam is driven through the pipe, the pipe expands, compressing the steam that comes through it. This pressure is the steam pressure. When the pipe is compressed by the steam blast, it expands back into the pipe, allowing the steam to be directed back into the cylinders. This steam blast must be from the bottom of the pipe, unless the boiler has a waste-gate, and is taking waste air from the cylinders through the same pipe.
As the steam pressure is increased in the pipe, the steam trap will expand, widening the pipe, and providing more space for the steam blast to push against the pipe. Eventually, the steam trap will grow large enough, and the steam pressure in the pipe will begin to exceed the internal pressure of the valve. This valve will not close under high pressure. The steam blast will then blow the valve completely off the pipe. The steam blast will explode into the air, carrying the pipe with it. The piston of the engine will not stop pushing the piston downward, since the steam trap has expanded.
This is a very dangerous situation. If the steam pressure is not controlled, a hole will form in the piping, and the steam pressure will rise. If the steam pressure rises too high, the pipe may collapse and the cylinder will seize. The cylinder will then blow out, throwing hot steam out. When the piston hits the throttle, it will push back and forth, making the throttle move downward and forwards, compressing and expanding the steam.
When the piston gets high enough, it will try to force the cylinder into the start position. This will also increase the steam pressure in the pipe. At this point, the valve will open, letting steam out of the pipe, and in the process blowing the valve. Steam pressure will build up in the pipe, and the steam pressure will reach a point where it is now difficult to control the valve.